Researches on the organic LEDs started by Pope, et al. in 1963 (J. Chem. Phys. 38 (1963) 2042). They used anthracene as a monolithic light emitting material, which could emit blue light under a high voltage. Since then, despite of improvements carried out by a few researchers (Phys. Rev. Lett. 14 (1965) 229; Sol. State Comm. 32 (1979) 683; Thin Solid Films 94 (1982) 476), the operating voltage was still too high, and the efficiency of energy conversion was still too low, preventing such materials from being used for practical purposes.
In 1987, Tang, et al. (Appl. Phys. Lett., 51 (1987) 914) used a vapor deposition technique to produce an organic LED with a structure of ITO/Diamine/Alq3/Mg:Ag, wherein ITO was a conductive transparent indium/tin oxide, and Alq3 was tris(8-hydroxyquinoline) aluminum. Because such an organic LED had an external quantum efficiency of 1% and a high brightness of 1000 cd/m2 (10V), researches and developments on the organic LEDs accelerate since then. Two years later, a research group in the Carvendish Laboratory of the Cambridge University in England used PPV as a light emitting material to make a LED having a structure of ITO/PPV/Ca, which emitted an olive color with a quantum efficiency of 0.05%, wherein ITO is a positive electrode, Ca is a negative electrode, and the PPV is poly(phenylene vinylene) (Nature, 347(1990) 539; U.S. Pat. Nos. 5,247,190 (1993); 5,425,125 (1995); 5,401,827 (1995)).
A primitive organic LED has a single organic layer, which is an organic light emitting layer disposed between a transparent electrode (as a positive electrode) and a metal electrode (as a negative electrode). In order to improve the light emitting efficiency of the organic LED, one LED can have two organic layers, wherein the first layer is a hole transporting layer and the second layer is the organic light emitting layer, or the first layer is the organic light emitting layer and the second layer is an electron transporting layer. Some LEDs may have three organic layers, which sequentially are the hole transporting layer, the organic light emitting layer, and the electron transporting layer. The light emitting process of such LEDs is described in the following: After the application of a positive bias, holes and electrons are separately emitted from the positive and the negative electrodes driven by an electric field resulting from the positive bias, which, after overcoming individual energy barriers, encounter each other in the light emitting layer and form excitons. The excitons rapidly decay radiantly back to the base state while emitting light. Such a LED is a type of Schottky characteristics.
Even though there are many blue LEDs have been proposed, only very few of them have a high brightness, excellent CIE coordinates, and a high efficiency. Among known OLED materials[1-6], Alq3 was the most popular dopant for green and red OLEDs. However, the energy gap between HOMO and LUMO of Alq3 is too small[7] such that Alq3 is not suitable to be used as a blue dopant. Therefore, for the development of full color displays with blue, green and red light, it was crucial to find a blue emitter that is stable and reliable and has a high efficiency.[8]
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